Abstract: Alterations in the salinity regimes of freshwater ecosystems may be contributing to global declines in freshwater biodiversity. Theory predicts that temperature should interact with salinity to influence fitness, but we know relatively little regarding how, or if, these two factors interact. Our objective was to assess how strongly salinity and temperature individually and jointly influence growth and survival of three stonefly species (Pteronarcys californica, Zapada cinctipes, and Yoraperla brevis). We selected these species for study based on their apparent sensitivity to salinity as inferred from survey data. We exposed the larvae of each species to 6 concentrations (0-1,000 mg/L) of a MgSO₄/CaSO₄ mixture and 6 constant temperatures (12-22 °C) during 2-6 month long laboratory experiments. Both treatment gradients covered much of the naturally occurring ranges of salinity and temperature observed across stream ecosystems including the upper limits thought to constrain the distributions of many aquatic insects. Several salts can contribute to salinity, but we used sulfate salts in our experiments because (1) they are thought to be more toxic to aquatic life than other common anions and (2) previous short-term studies have shown that the effect of sulfate salts on survival can be dependent on temperature. For each treatment combination, we recorded survival at weekly intervals and estimated individual growth as mf-mi/d, where mf = mean dry mass of individuals surviving to the end of the experiment, mi = mean dry mass at the start of the experiment, and d = number of days of exposure. We expected to see an interactive effect of temperature and sulfate on both survival and growth, with both responses lowest at the highest sulfate and temperature treatments. Both Zapada and Yoraperla experienced nearly 100% mortality at 20 and 22 °C, but neither survival nor growth varied across the sulfate gradient. Survival of Pteronarcys did decrease with increasing sulfate concentration, with lowest (50-60%) average survival occurring in the 600, 800, and 1,000 mg/L sulfate treatments compared with 74-79% survival in < 600 mg/L treatments. We found no evidence that sulfate and temperature interacted to influence either survival or growth for any species. Our results suggest that salinity may not be as important of a driver of species distributions of aquatic insects as inferred from survey data, and tolerance limits derived from survey data should therefore be used with caution in setting water quality standards for the protection of aquatic life.
